In January he will have 18 months on his contract left. We won't get close to £50m at that point, unless Brazil have won the World Cup and he wins the golden boot.
Also, if we were to refuse to sell now, January there might not even be a market for him. Clubs will know they can wait 6 months and Everton will be even more desperate to sell him with 12 months left on his contract.
We can't afford to watch him walk away on a free when there is £50m on offer now. The club will know this and I expect Richarlison has made it clear that he won't sign a new deal and he wants to leave.